1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) cellar system, in particular, a CDMA cellar system, an optimum path detecting method, an apparatus thereof, a mobile portable apparatus, and a base station apparatus that allow a path timing to be obtained corresponding to the maximum reception level of reception data.
2. Description of the Related Art
In a digital mobile telephone system and a portable telephone system (cellar system) that use the CDMA system, RAKE/diversity receiving technologies and transmission power controlling technologies have been widely used as a CDMA cellar system in multi-path fading environments because of high secrecy, interference resistance, high communication capacity, and high quality communication.
Amount them, a reception path timing searching technology is important. The searching accuracy of the reception path timing largely affects characteristics of the RAKE/diversity receiving process of the CDMA cellar system.
As a first related art reference, a spread spectrum receiving apparatus disclosed in Japanese Patent Laid-Open Publication No. 8-340316 will be described. FIG. 1 is a block diagram showing principal portions of a spread spectrum receiving apparatus. In FIG. 1, a received spread spectrum signal is supplied to a frequency converting circuit 121. The frequency converting circuit 121 converts the spread spectrum signal into a low frequency signal. The low frequency signal is supplied to a first multiplying unit 122 of a first inversely spreading circuit. The first multiplying unit 122 multiplies the low frequency signal by a spread code P2 generated by a shift register 132 (that will be described later). An output signal of the first multiplying unit 122 is supplied to a phase comparing circuit 124.
The phase comparing circuit 124 compares the phase of the output signal of the first multiplying unit 122 with the phase of an output signal of a VCO 130 that varies the frequency of a generated signal. An output signal of the phase comparing circuit 124 is supplied to a low pass filter LPF 126. The low pass filter LPF 126 smooths the output signal of the phase comparing circuit 124. An output signal of the low pass filter LPF 126 is supplied as a control signal to the VCO 130. The first multiplying unit 122, the phase comparing circuit 124, the LPF 126, the VCO 130, a frequency dividing circuit 127, a spread code generating circuit 123, and the shift register 132 compose a phase synchronizing circuit (namely, a phase locked loop: PLL). The PLL operates so that the phase difference between two input signals of the phase comparing circuit 124 becomes zero.
The output signal of the VCO 130 is also supplied to the frequency dividing circuit 127 in addition to the phase comparing circuit 124. The frequency dividing circuit 127 divides the frequency of the output signal of the VCO 130. The spread code generating circuit 123 generates a spread code P0 corresponding to an output signal of the frequency dividing circuit 127. In addition, the shift register 132 generates a plurality of spread codes P1 to P4 with difference phases corresponding to the spread code P0. The shift register 132 is composed of for example four staged registers. The spread code P0 received from the spread code generating circuit 123 is successively transferred from the first staged register to the fourth staged register corresponding to a clock signal that is the output signal of the VCO 130. An output signal of the first staged register is the spread code P4. Output signals of the second to fourth staged registers are the spread codes P3, P2, and P1, respectively. The spread codes P1, P2, P3, and P4 delay from the spread code P0 by four clock pulses, three clock pulses, two clock pulses, and one clock pulse of the output signal of VCO 130, respectively.
The spread code generating circuit 123 is composed of for example a shift register and an exclusive OR gate. The spread code generating circuit 123 is a well-known circuit that generates an M code sequence corresponding to the clock signal that is the output signal of the VCO 130. The spread codes P1, P2, P3, and P4 synchronize with the output signal of the VCO 130. The spread codes P1, P2, P3, and P4 are supplied to second inversely spreading circuits 133, 134, 135, and 136, respectively. The second inversely spreading circuits 133 to 136 inversely spread the spread spectrum codes.
Output signals of the second inversely spreading circuits 133 to 136 are supplied to a level detecting circuit 137. The level detecting circuit 137 detects levels of the output signals of the second inversely spreading circuits 133 to 136 by envelop detecting method and extracts correlations between the spread spectrum signals and spread codes. Output signals of the level detecting circuit 137 are supplied to a determining circuit 138. The determining circuit 138 determines a signal with the highest level of the output signals of the second inversely spreading circuits 133 to 136. An output signal of the determining circuit 138 is supplied to a switching circuit 139. The switching circuit 139 selects a signal with the highest level from the output signals of the second inversely spreading circuits 133 to 136 corresponding to the determined result of the determining circuit 138.
An output signal of the switching circuit 139 is supplied to a BPF 128. The BPF 128 limits the frequency band of the output signal of the switching circuit 139. An output signal of the BPF 128 is supplied to a demodulating circuit 129. The demodulating circuit 129 demodulates the output signal of the BPF 128. The plurality of second inversely spreading circuits detect a level with the highest correlation. Thus, the synchronization of spread spectrum codes can be securely acquired and tracked.
Next, a second related art reference disclosed in Japanese Patent Laid-Open Publication No. 7-193525 will be described. The second related art reference relates to a synchronizing method for a radio communication network, in particular, to a synchronizing method for a radio communication network of which each repeating station repeats a synchronous signal at a transmission timing fixedly assigned with an ultra-frame using frequency hopping type spread spectrum modulation (FH-CDMA) and a time division multiple access (TDMA). In the second related art reference, the reception levels of synchronous signals received from individual repeating stations are successively compared for individual ultra-frames. The transmission timing with the maximum reception level is stored. In the next ultra-frame, an operation corresponding to the synchronous signal at the transmission timing stored in the preceding ultra-frame is repeated for individual ultra-frames. Thus, an adjacent station with the highest reception level can be tracked. Consequently, a signal can be synchronously and stably received.
Next, a third related art reference disclosed in Japanese Patent Laid-Open Publication No. 9-261128 will be described. The third related art reference relates to a synchronizing apparatus disposed in a spread spectrum communication receiver for use with a digital mobile radio communication system. The synchronizing apparatus comprises a DLL for synchronously tracking a PN (Pseudo-Noise) signal against sampling data received from an A/D converter that converts a reception signal into a digital signal, a searching unit for searching a path from which a reception signal with the maximum power is obtained, and a data demodulating correlator for inversely spreading and demodulating the reception signal.
The searching unit has a searching PN generator for generating a PN phase of the PN signal, a searching correlator for correlating sampling data with the PN signal and outputting correlation value data, a data buffer for storing the correlation value data, a path searching unit for searching correlation value data of a path with the maximum power from the data buffer and outputting the PN path of the searched correlation value data, a means for outputting the PN phase of the PN signal received from the path searching unit to the DLL and the data demodulating correlator, and a PN signal switching means for switching the PN phase of the PN signal that is output from the PN signal outputting means to the phase of the path with the maximum power wherein the PN phase of the PN signal that is output to the DLL is the same as the phase of the data demodulating PN signal so that the phase of the path with the maximum power is always maintained.
In such a structure, with a means for switching the phase of the PN signal that is output to the DLL to the phase of a path with the maximum power in such a manner that the phase of the PN signal that is output to the DLL matches with or is independent from the phase of the PN signal to be demodulated in the case the level of a path tracked by the DLL (Delay Locked Loop) decreases due to fading and the synchronization of the DLL is lost, before the synchronization of the DLL is lost, the phase of the PN signal that is output to the DLL is switched to the phase of another path with the highest power.
As conventional reception path timing searching methods, in the first related art reference, the output signal of the second inversely spreading circuit is selected corresponding to an output signal with the highest correlation value. In the second related art reference, a signal with the highest reception level transmitted from an adjacent station is always tracked. In the third related art reference, the phase of a PN signal is always switched to the phase of a path with the maximum power. According to the third related art reference, the DLL obtains a correlation of a predetermined number of chips in a predetermined delay time period is obtained. Next, the delay time period is repeatedly varied by a predetermined value (for example, xc2xd chip). A point with the largest correlation is treated as a reception timing position at which a signal is received. This method is referred to as sliding correlation calculation searching method.
However, in SSA (Serial Search Acquisition) method as the sliding correlation calculation searching method, even if the reception level of a signal fluctuates due to a multi-path fading environment and deteriorates, a reception path timing is searched corresponding to the reception signal at the point. Thus, since the reception path is incorrectly detected, the reception accuracy deteriorates.
When the reception state deteriorates in the multi-path environment and an reception error takes place, although reception data and reception level (RSSI: Receive Signal Strength Indicator) fluctuate, the reception data is demodulated corresponding to an established acquisition condition. Thus, the reliability of the established reception tracking technology may be lost. Consequently, data cannot be received at a low error rate.
The present invention is made from the above-described point of view. An object of the present invention is to provide a cellar system, a mobile portable apparatus, a base station apparatus and an optimum path detecting method which a reception path timing is searched with reception data in a large reception level and reception data is correlated and combined corresponding to the reception paths. Further, an another object of the present invention is to provide the apparatus which an incorrect detection of a reception path due to the fluctuation of a reception level can be suppressed. Furthermore, an another object of the present invention is to provide the apparatus which in a multi-path fading environment, a reception signal can be stably and accurately obtained.
A first aspect of the present invention is a cellar system using code divisional multiple access (CDMA) system, comprising a plurality of finger circuits, and a search engine portion having a reception level measuring portion for detecting reception levels from reception signals and comparing the reception levels with a predetermined threshold value, a plurality of inversely spreading portions for multiplying the reception signals by spread codes, an inner memory for storing correlation signals received from the plurality of inversely spreading portions, and a reception path timing generating portion for detecting reception paths from output signals of the inner memory and generating a path timing, wherein the search engine portion determines whether or not to output a correlation signal of the inner memory to the reception path timing generating portion.
The reception levels of the reception signals are measured for each frame. When the reception levels of the reception signals in the current frame are lower than the predetermined threshold value, a correlation signal in the preceding frame is output from the inner memory.
The plurality of finger circuits inversely spread the reception signals corresponding to the reception paths detected by the search engine portion and synchronously adds correlation output signals of the plurality of finger circuits.
A second aspect of the present invention is a mobile portable apparatus using code divisional multiple access (CDMA) system, comprising a demodulating portion for demodulating reception signals to base-band signals, and a search engine portion for selecting reception paths corresponding to multi-paths, the searching engine portion having a reception level measuring portion for detecting reception levels from reception signals and comparing the reception levels with a predetermined threshold value, a plurality of inversely spreading portions for multiplying the reception signals by spread codes, an inner memory for storing correlation signals received from the plurality of inversely spreading portions, and a reception path timing generating portion for detecting reception paths from output signals of the inner memory and generating a path timing, wherein the search engine portion determines whether or not to output a correlation signal of the inner memory to the reception path timing generating portion.
A third aspect of the present invention is a base station apparatus of a cellar system using code divisional multiple access (CDMA) system, comprising an antenna for transmitting and receiving a signal to/from a mobile portable apparatus, a radio portion for transmitting and receiving a signal to/from the antenna, and a baseband portion for transmitting and receiving a signal to/from the radio portion, wherein the baseband portion has a spreading portion for spreading a transmission digital signal, a RAKE receiving portion for receiving signals corresponding to multi-paths, and a search engine portion for detecting reception paths, wherein the search engine portion has a reception level measuring portion for detecting reception levels from reception signals and comparing the reception levels with a predetermined threshold value, a plurality of inversely spreading portions for multiplying the reception signals by spread codes, an inner memory for storing correlation signals received from the plurality of inversely spreading portions, and a reception path timing generating portion for detecting reception paths from output signals of the inner memory and generating a path timing, wherein the search engine portion determines whether or not to output a correlation signal of the inner memory to the reception path timing generating portion.
A fourth aspect of the present invention is an optimum path detecting method used for code divisional multiple access (CDMA) system for demodulating reception signals to baseband signals and causing a search engine portion to select reception paths corresponding to multi-paths from the baseband signals, comprising the steps of detecting reception levels of the baseband signals, comparing the reception levels with a predetermined threshold value, multiplying the baseband signals by a spread code so as to inversely spread the reception signals, storing correlation signals corresponding to the inversely spread results to an inner memory, detecting a reception path from an output signal of the inner memory, generating a path timing corresponding to the reception path, and selectively outputting the correlation signal from the inner memory corresponding to the compared results.
A fifth aspect of the present invention is an optimum path detecting method used for code divisional multiple access (CDMA) system, comprising the steps calculating correlation values of reception signals at predetermined intervals, comparing the correlation values with a predetermined threshold value, determining whether to select a reception path corresponding to a peak position of the correlation value of the current reception signal or the just preceding reception signal corresponding to the compared results.
A sixth aspect of the present invention is an optimum path detecting apparatus used for code divisional multiple access (CDMA) system, comprising a means for detecting correlations of reception signals at predetermined intervals, a means for detecting the levels of the reception signals, a means for detecting a reception path corresponding to a peak position of a correlation value of the current reception signal or the just preceding reception signal corresponding to the detected levels.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawings.